As disclosed in JP 4232471 B2, there is conventionally known a steering control apparatus that provides assistance to steering force applied to a steering member of a vehicle. The steering control apparatus in JP 4232471 B2 performs a first control and a second control. The first control is exercised to drive a steering assist motor, which generates an assist steering torque, on the basis of a detected steering torque. Hereinafter, the assist steering torque is referred to as an assist torque for simplification. The second control is exercised to drive the steering assist motor on the basis of the difference between the detected steering torque and a target steering torque. The target steering torque is estimated based on a detected steering angle and a detected vehicle speed.
As a result, the steering control apparatus is possible to switch an operation between the first control and the second control. As described above, the first control is exercised to drive the vehicle by providing a steering torque based on road surface conditions. The second control is exercised to drive the vehicle by providing a predetermined steering torque irrespective of the road surface conditions. Further, the second control is exercised to generate the target steering torque by using a standard steering model defining the relationship between the steering angle and steering torque of the steering member. The second control is also exercised to vary mechanical impedance, which characterizes the standard steering model, on the basis of the steering angle and a vehicle speed.
It is known that a driver is capable of determining vehicle conditions from steering feel by tactile sense. However, the first control, which is based only on road surface load, may fail to provide a finely adjusted steering feel corresponding to the steering operation performed by the driver.
Further, the second control refers to the steering angle and the vehicle speed as parameters for varying the mechanical impedance. Herein, the mechanical impedance is transferred to the driver as the steering feel. Therefore, in various steering states that may occur during an actual driving, the second control may also fail to provide a finely adjusted steering feel corresponding to the steering operation performed by the driver. Herein, various steering states that may occur during an actual driving includes, for example, rotating a steering wheel toward a direction in normal rotation speed, rotating back the steering wheel from the rotated direction, retaining or holding the steering wheel without rotation, or rapidly rotating the steering wheel. Hereinafter, rotating back of the steering wheel from the rotated state may also be referred to as returning the steering wheel for distinguishing the rotating from the rotating back.